The invention relates to a tool for conducting performance analysis on optical transmission systems, and more particularly to a method for defining custom eye pattern masks for sensing signal distortions.
Eye masks consist of regions or polygons that collectively express constraints on the distortion of signal waveforms such as may occur in transmission links or on the signal waveforms generated by optical transmitters. The eye mask quickly makes several measurements in one comparison, and also gives valuable visual feedback to the success or failure of the test. Virtually every industry standard on compliance testing for optical transmitters defines a specific eye-mask.
Distortion is any inaccurate reproduction of a generated signal which is referred to system elements placed anywhere in a transmission link. Distortion can be measured by comparing the differences in wave shape between the original signal and that of the signal after it has traversed the transmission link. Optical signals can suffer degradation in transmission from noise, inter-symbol interference, fiber dispersion etc.
As data transmission rates for signals increase, the requirement for faster and more sensitive transmission systems increases. The extent of signal degradation can be viewed via an eye closure diagram which is the graphic pattern produced on a digital oscilloscope when a baseband signal is applied to the vertical input of the digital oscilloscope while the symbol rate serves to trigger the time base. For a binary signal, the eye closure diagram has a single xe2x80x9ceyexe2x80x9d which is open or closed to a degree determined by the amount of signal degradation. An open eye pattern is desired. Changes in the xe2x80x9ceyexe2x80x9d size indicate intersymbol interference, amplitude irregularities, or timing problems such as jitter, depending on the signal that is being measured.
To facilitate the use of an eye closure diagram, a reference eye pattern mask is typically established on an eye closure diagram. A reference eye pattern mask typically comprises a set of pre-defined regions such as polygons on digital oscilloscope screen or other visual display and effectively defines regions of exclusion. When the waveforms of the eye closure diagram intrude into the pre-defined region comprising a reference eye pattern mask, a mask violation occurs indicating problems with the quality of the sampled optical signal.
Standard reference patterns of eye masks for non-return to zero (NRZ) binary encoding are available in commercial measurement and testing products, while no standard reference patterns are yet available for return to zero (RZ) binary encoding. Many test scenarios require the use of custom masks, in order to create more stringent tolerances or to emphasize specific waveform characteristics (e.g. soliton characteristics). Presently, custom reference eye mask creation is only possible by having the user geometrically define the mask region by specifying the vertex coordinates for all the regions making up the reference eye mask that is used. This is a cumbersome, non-intuitive and difficult to control method for generating precise custom reference eye mask patterns.
In accordance with the present invention, eye mask patterns can be defined by physical parameters that relate directly to key waveform parameters including jitter, transition times and signal strength. The parametric eye mask representation conveys a physical meaning to eye mask descriptions and the eye mask creation process. Depending on the method of implementation, the parametric eye mask representation allows user friendly interactive eye mask creation using, for example, a personal computer, work station or digital oscilloscope. Parametric representations for Non-Return-to-Zero (NRZ) eye masks and for Return-to-Zero (RZ) eye masks may be created. NRZ and RZ are fundamental encoding schemes and the eye diagrams for other common coding schemes such as Manchester, AMI and CMI reduce to the NRZ type. The parameter set for NRZ eye masks is a subset of the parameter set used to describe RZ masks, as NRZ masks are symmetric with respect to the horizontal axis while RZ masks are asymmetric with respect to the horizontal axis.